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SubjectRe: [RFC] x86, ia32entry: Use sysretl to return from sysenter
On Fri, Mar 27, 2015 at 10:54 PM, Andy Lutomirski <luto@kernel.org> wrote:
> --- a/arch/x86/ia32/ia32entry.S
> +++ b/arch/x86/ia32/ia32entry.S
> @@ -180,28 +180,34 @@ sysenter_dispatch:
> testl $_TIF_ALLWORK_MASK, ASM_THREAD_INFO(TI_flags, %rsp, SIZEOF_PTREGS)
> jnz sysexit_audit
> sysexit_from_sys_call:
> + /*
> + * NB: sysexit is not obviously safe for 64-bit kernels -- an
> + * NMI between sti and sysexit has poorly specified behavior,
> + * and and NMI followed by an IRQ with usergs is fatal. So
> + * we just pretend we're using sysexit but we really use
> + * sysretl instead.
> + *
> + * This code path is still called sysexit because it pairs
> + * with sysenter and it uses the sysenter calling convention.
> + */
> andl $~TS_COMPAT,ASM_THREAD_INFO(TI_status, %rsp, SIZEOF_PTREGS)
> - /* clear IF, that popfq doesn't enable interrupts early */
> - andl $~0x200,EFLAGS(%rsp)
> - movl RIP(%rsp),%edx /* User %eip */
> - CFI_REGISTER rip,rdx
> + movl RIP(%rsp),%ecx /* User %eip */
> + CFI_REGISTER rip,rcx
> RESTORE_RSI_RDI

I think you need to replace
RESTORE_RSI_RDI with RESTORE_RSI_RDI_RDX

> - /* pop everything except ss,rsp,rflags slots */
> - REMOVE_PT_GPREGS_FROM_STACK 3*8
> + xorl %edx,%edx

Why do you clear %edx?


> xorq %r8,%r8
> xorq %r9,%r9
> xorq %r10,%r10
> - xorq %r11,%r11
> - popfq_cfi
> + movl EFLAGS(%rsp),%r11d /* User eflags */
> /*CFI_RESTORE rflags*/
> - popq_cfi %rcx /* User %esp */
> - CFI_REGISTER rsp,rcx
> TRACE_IRQS_ON
> /*
> - * 32bit SYSEXIT restores eip from edx, esp from ecx.
> - * cs and ss are loaded from MSRs.
> + * Sysretl works even on Intel CPUs. Use it in preference to sysexit,
> + * since it avoids a dicey window with interrupts enabled.
> + * CS and SS are loaded from MSRs.

Please do not remove the mini-doc which says what is restored from where.
These instructions are not that obvious. I propose:

* 64bit->32bit SYSRET restores eip from ecx,
* eflags from r11 (but RF and VM bits are forced to 0),
* CS and SS are loaded from MSRs. CS and SS are loaded from MSRs.



Or maybe just

...
sysexit_from_sys_call:
/*
* Sysretl works even on Intel CPUs. Use it in preference to sysexit,
* since it avoids a dicey window with interrupts enabled.
*/
jmp to sysretl_from_sys_call



and remove the entire "sysexit tail" code path?


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